By Pavel Polityuk and Max Hunder KYIV (Reuters) – Russian forces on Wednesday fought their way into the centre of the Ukrainian industrial city of Sievierodonetsk and appeared to be close to claiming a big prize in their offensive in the eastern Donbas region. But in a boost for Ukraine, locked in a grinding struggle against the invading army, the United States announced a new $700 million weapons package for Kyiv which will include advanced rocket systems capable of hitting targets up to 80 km (50 miles) away. Moscow accused the United States of adding “fuel to the fire” and Russian Foreign M…